This invention relates to an Fexe2x80x94Crxe2x80x94Ni alloy which is required to be nonmagnetic and is used in electron gun electrodes, and specifically relates to an alloy with improved press forming properties for drawing.
In general, electron gun electrodes used in color picture tubes and the like are produced by drawing a nonmagnetic Fexe2x80x94Crxe2x80x94Ni stainless steel material with a thickness of 0.05 to 0.5 mm into a predetermined shape using press forming. In order to improve the formability for drawing, in particular, to facilitate burring (working in which a circular hole is formed and the circumference thereof is projected like a cylinder), improvement in degree of rolling reduction and annealing conditions has been proposed in Japanese Patent Application, First Publication, No. 257253/94. Japanese Patent Application, First Publication, No. 205453/96 proposes a method in which press forming properties are improved by limiting center line average height and maximum height of surface roughness in press forming using a low viscosity lubricating oil, which is easy to degrease and has been used to increase production efficiency. Japanese Patent Application No. 283039/97 demonstrates that burrs remaining in press punching a through hole relates to cracks in burring, and proposes a method in which burring properties are improved by suitable amounts of S being contained to improve punching properties and minute amounts of the elements are controlled to improve the formability for drawing.
According to the rapid advances of finer and brighter picture tubes for computers in recent years, requirements on focusing characteristics of the electron guns has become more severe. Therefore, the requirement on materials is necessary to have not only high precision formability for the large lens diameter electrodes but also good formability for high speed press forming. As a result, the prior art alloys have not been adequate since cracks occur on drawing surfaces.
The present invention has been made to complete the above situation. An object of the invention is to provide an Fexe2x80x94Crxe2x80x94Ni alloy for electron gun electrodes, having superior formability for drawing, which has been more severe in recent years, in particular, having superior surface qualities after drawing.
The inventors have extensively studied the surface conditions of materials to complete the problems. As a result, the inventors have found that the formability for drawing is influenced by the degree of sharpness of projections in surface profile. In particular, the inventors have found that the formability for drawing is inferior and surface cracks in drawing readily occur when the ends of the projections are sharp and the intermediate portion (valley) between the projections is deep and steep. In particular, it has been estimated that cracks would surely occur when the valley is deep and steep and when foreign particles such as inclusions are present at the bottom of the valley. The inventors have made the invention by representing the degree of the sharpness of the projections by kurtosis Kr and analyzing the relationship between the kurtosis Kr and the formability for drawing. The kurtosis Kr is represented by the following formula (1).
Kr=xcexa3(Yi/Rq)4/Nxe2x80x83xe2x80x83(1)
Wherein yi is roughness profile, Rq is root mean square roughness, and N is number of samples.
This invention provides an Fexe2x80x94Crxe2x80x94Ni alloy for electron gun electrodes comprising: 15 to 20% Cr; 9 to 15% Ni; 0.12% or less C; 0.005 to 1.0% Si; 0.005% to 2.5% Mn; 0.03% or less P; 0.0003 to 0.0100% S; 2.0% or less Mo; 0.001 to 0.2% Al; 0.003% or less O; 0.1% or less N; 0.1% or less Ti; 0.1% or less Nb; 0.1% or less V; 0.1% or less Zr; 0.05% or less Ca; 0.02% or less Mg; and the balance Fe and inevitable impurities by weight, and the alloy having a surface roughness satisfying the following formula (2) when kurtosis in the rolling direction and kurtosis in the transverse direction to the rolling direction in surface roughness of the alloy are respectively defined as Kr0 and Kr90.
Kr0xe2x89xa64, Kr90xe2x89xa64xe2x80x83xe2x80x83(2)
The reasons for the above limitations in the surface roughness and the alloy composition in the Fexe2x80x94Crxe2x80x94Ni alloy for electron gun electrodes will be explained together with the effects of the present invention. In the following explanation, xe2x80x9c%xe2x80x9d means xe2x80x9cweight %xe2x80x9d.
(Kr0, Kr90): The above-mentioned kurtosis range has been found by the inventors performing quantity analysis. According to the research by the inventors, if Kr0 and Kr90 are more than 4, a large number of high ridges and deep valleys with very sharp shapes exist in the surface roughness profile, and as a result, cracks occur on the drawn surface. Therefore, Kr0 and Kr90 are restricted to 4 or less.
(Cr): Electron gun electrodes are essentially required to be nonmagnetic. Normally, permeability is required to be 1.005 or less for them to be nonmagnetic. In order to meet the requirement, the content of Cr is restricted to within the range of 15 to 20%. A more preferable range for the Cr content is from 15 to 17%.
(Ni): If the Ni content is less than 9%, magnetic characteristics increase. If the Ni content exceeds 15%, the material cost increases too much. Hence, the Ni content is restricted to within the range of 9 to 15%.
(C): If the C content exceeds 0.12%, a large amount of carbide is formed, thereby the formability for drawing is inferior, and hence, the C content is restricted to 0.12% or less.
(Si): Si is added for deoxidation. If the Si content is less than 0.005%, the effect as a deoxidizer cannot be obtained. On the other hand, if the Si content exceeds 1.0%, the formability is inferior. Hence, the Si content is restricted to within the range of 0.005 to 1.0%.
(Mn): Mn is added for deoxidation and formation of MnS. If the Mn content is less than 0.005%, these effects are not expected. If the Mn content exceeds 2.5%, the hardness of the alloy increases, thereby the formability for drawing is inferior. Hence, the Mn content is restricted to within the range of 0.005 to 2.5%.
(P): If the P content exceeds 0.03%, the formability for drawing is inferior. Hence, the P content is restricted to 0.03% or less.
(S): When S is contained in an appropriate amount, S forms MnS together with Mn, so that the forming of burrs is inhibited in press punching a hole and cracks in burring is inhibited. If the S content is less than 0.0003%, such effects are not expected. If the S content exceeds 0.0100%, coarse MnS is formed, thereby the formability for drawing is inferior. Hence, the S content is restricted to within the range of 0.0003 to 0.0100%.
(Mo): Since Mo improves corrosion resistance, Mo can be advantageously added when special corrosion resistance is required. However, if the Mo content exceeds 2.0%, the formability for drawing is inferior. Hence, the Mo content is restricted to 2.0% or less.
(Al): Al is added for deoxidation, which is effective with an Al content of 0.001% or more. If the Al content exceeds 0.2%, the formability for drawing is inferior. Hence, the Al content is restricted to within the range of 0.001 to 0.2%.
(O): When an exceeding large amount O is contained, the amount of oxide-type inclusions increase, thereby the formability for drawing is inferior. Hence, the O content is restricted to 0.003% or less.
(N): When the N content exceeds 0.1%, the formability is inferior. Hence, the N content is restricted to 0.1% or less.
(Ti): Ti forms carbides, sulfides, oxides and nitrides, thereby the formability for drawing is inferior. Hence, the Ti content is restricted to 0.1% or less. A more preferable range for the Ti content is 0.02% or less.
(Nb): Nb forms carbides, sulfides, oxides and nitrides, thereby the formability for drawing is inferior. Hence, the Nb content is restricted to 0.1% or less. More preferable range of the Nb content is 0.02% or less.
(V): V forms carbides and nitrides, thereby the formability for drawing is inferior. Hence, the V content is restricted to 0.1% or less. A more preferable range for the V content is 0.02% or less.
(Zr): Zr forms oxides, thereby the formability for drawing is inferior. Hence, the Zr content is restricted to 0.1% or less. A more preferable range for the Zr content is 0.02% or less.
(Ca): Ca forms sulfides and oxides, thereby the formability for drawing is inferior. Hence, the Ca content is restricted to 0.05% or less. A more preferable range for the Ca content is 0.01% or less.
(Mg): Mg forms oxides, thereby the formability for drawing is inferior. Hence, the Mg content is restricted to 0.02% or less. A more preferable range for the Mg content is 0.005% or less.
The inventors have found that the formability for drawing is inferior when the difference is large between the rolling direction and the transverse direction to the rolling direction in a horizontal cross section in the surface of the material. In particular, the inventors have paid attention to root mean square inclination of the profile of the horizontal cross section, which shows the standard deviation in the inclination of the slant between the ridges and valleys on the surface of the material. The inventors demonstrated the difference between the rolling direction and the transverse direction to the rolling direction in a profile of the horizontal cross section, as a ratio of the root mean square inclination xcex94q in the rolling direction and the root mean square inclination xcex94q in the transverse direction to the rolling direction. They have studied the relationship between the ratio and the formability for drawing. As a result, they found that when the large value is obtained by dividing the root mean square inclination in the rolling direction with the root mean square inclination in the transverse direction to the rolling direction, the difference between lubricating properties in both directions is large, and the formability for drawing is inferior. The root mean square inclination xcex94q is shown by the following formula (3).
xcex94q={xcexa3(xcex94y/xcex94x)2/N}xc2xdxe2x80x83xe2x80x83(3)
Wherein xcex94y is the vertical increase with respect to a horizontal small deviation.
According to research by the inventors, when the root mean square inclination in the rolling direction and the root mean square inclination in the transverse direction to the rolling direction in the surface roughness of the material are respectively defined as xcex94q0 and xcex94q90, it has been demonstrated that if xcex94q0/xcex94q90 is larger than 4, the difference between lubricating properties of the rolling direction and the transverse direction to the rolling direction is large, and the formability for drawing is inferior. Therefore, xcex94q0/xcex940 is preferably 4 or less.
It should be noted that if the cleanliness based on JIS G0555 of the alloy exceeds 0.03%, the formability for drawing, in particular, the formability for both deep drawing and high burring, is inferior. Therefore, the cleanliness of the alloys should be 0.03% or less.
Preferred embodiments of the invention will be described hereinafter.
In order to obtain the above-mentioned kurtosis and the root mean square inclination in a horizontal cross section, a material subjected to the final rolling into the required thickness may be mechanically polished with a fabric containing abrasives or SiC powder having various grain sizes. Alternatively, the kurtosis and the root mean square inclination in a horizontal cross section may be controlled by selecting the surface roughness of the matte roll used in the finish rolling.